Elimination of vapour anaesthetics from patients after surgical procedures

ABSTRACT

A breathing circuit system for ventilating an anaesthetized patient. The system comprises a standard primary circle anaesthetic circuit which itself comprises a one-way inspiratory limb for delivering re-breathed gas and a one-way expiratory limb for accepting expired gas. The breathing circuit system also includes a supplementary respiratory circuit which solely supplies non-rebreathed gas and comprises a source of non-rebreathed, substantially carbon dioxide-free gas, a non-recreated fresh gas reservoir for storing fresh gas, a source of non-rebreathed reserve gas containing carbon dioxide, and a gas delivery conduit. Disposed in communication with the inspiratory limb is a non-re-breathing valve, while disposed in communication with both the inspiratory limb and the delivery conduit is a three-way respiratory valve for selectively permitting passage of gas from the inspiratory limb or from the delivery conduit.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] (Not Applicable)

STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

[0002] (Not Applicable)

FIELD OF THE INVENTION

[0003] The purpose of this invention is to provide a simple breathingcircuit that can, for example, be added to a standard circle anaestheticcircuit known to persons skilled in the art to hasten recovery ofpatients administered vapour anaesthetics prior to an operation.

[0004] This invention also relates to the use of the breathing circuitin hastening the recovery of patients who have been administered vapouranaesthetics prior to surgical operation.

[0005] This invention also relates to methods of treatment of patientsto hasten their recovery from administration of the vapour anaestheticsto them prior to surgical procedures.

BACKGROUND OF THE INVENTION

[0006] Physiology

[0007] Venous blood returns to the heart from the muscles and organsdepleted of oxygen (O₂) and full of carbon dioxide (CO₂). Blood fromvarious parts of the body is mixed in the heart (mixed venous blood) andpumped to the lungs. In the lungs the blood vessels break up into a netof small vessels surrounding tiny lung sacs (alveoli). The net ofvessels surrounding the alveoli provides a large surface area for theexchange of gases by diffusion along their concentration gradients. Aconcentration gradient exists between the partial pressure of CO₂(PCO₂)in the mixed venous blood (PvCO₂) and the alveolar PCO₂. The CO₂diffuses into the alveoli from the mixed venous blood from the beginningof inspiration until an equilibrium is reached between the PvCO₂ and thealveolar PCO₂ at some time during the breath. When the subject exhales,the end of his exhalation is considered to have come from the alveoliand reflect the equilibrium concentration between the capillaries andthe alveoli; the PCO₂ in this gas is called end-tidal PCO₂ (PETCO₂).

[0008] When the blood passes the alveoli and is pumped by the heart tothe arteries it is known as the arterial PCO₂ (PaCO₂). The arterialblood has a PCO₂ equal to the PCO₂ at equilibrium between thecapillaries and alveoli. With each breath some CO₂ is eliminated andfresh air containing little CO₂ (assumed to be O) is inhaled and dilutesthe residual alveolar PCO₂, establishing a new gradient for CO₂ todiffuse out of the mixed venous blood into the alveoli. The rate ofbreathing, or ventilation (V), usually expressed in L/min, is exactlythat required to eliminate the CO₂ brought to the lungs and maintain anequilibrium PCO₂ (and PaCO₂) of approximately 40 mmHg (in normalhumans). When one produces more CO₂ (e.g. as a result of fever orexercise), more CO₂ is produced and carried to the lungs. One then hasto breathe harder (hyperventilate) to wash out the extra CO₂ from thealveoli, and thus maintain the same equilibrium PaCO₂. But if the CO₂production stays normal, and one hyperventilates, then the PaCO₂ falls.

[0009] It is important to note that not all V contributes to blowing offCO₂. Some V goes to the air passages (trachea and bronchi) and alveoliwith little blood perfusing them, and thus doesn't contribute to blowingoff CO₂. That the portion of V that goes well perfused alveoli andpaticipates in gas exchange is called the alveolar ventilation (VA).

[0010] There are a number of circumstances in therapeutic medicine andresearch where we want the subject to breath harder but not to exchangehis PaCO₂ (see Table 1). TABLE I Method of Type of InvestigationReference adjustment Source of CO₂ Respiratory muscle fatigue 5 M R 12 ME 7 M R Respiratory muscle training 2 M R 3 M R Increased V duringanaesthesia 6 M R Carotid chemoreceptor function 8 M E 1 M E Effect ofhypoxia on 10 M E symphathetic response 4 M E Control of respiration 9 AE Tracheobronchial tone 11 M E

[0011] Table 1:

[0012] Title: Summary of previous studies attempting to maintainconstant PETCO₂ during hyperopia

[0013] Legend: Method of adjustment of inspired PCO₂:M=manual;A=automated. Source of CO₂:R=rebreathing; E=external.

[0014] CO₂ tensions. Can. J. Anaesth. 43 (8):862-6, 1996.

[0015] This requires compensating for excess ventilation by inhaling CO₂either from exhaled gas or some external source. The amount of CO₂required to be inhaled needs to be adjusted manually or by an automatedservo-controlled mechanism, depending on how fine the control of PaCO₂is required. The input signal is the PETCO₂. Stability of PaCO₂ dependson the variability of CO₂ production and ventilation on the one hand,and the ability of a system to compensate for this variability on theother.

[0016] The termination of the anaesthetic effects of intravenouslyadministered drugs depends on metabolism and redistribution. Therecovery time from anaesthesia is therefore determined by the drug'spharmacology and cannot be accelerated.

[0017] This is not so for inhaled anaesthetic vapours. The uptake andelimination of anaesthetic vapours is predominantly through the lungs.The partial pressure of an anaesthetic vapour in the blood going to thebrain is dependent upon the equilibrium of vapour between the blood andthe lungs. The concentration of vapour in the lungs in turn is dependenton the concentration of vapour in the inhaled gas, the rate ofbreathing, and the rate of transfer of gas between the lung and theblood. The newer anaesthetic agents desflurane and sevoflurane have verylow blood solubility. Therefore the amount of drug transferred betweenthe lungs and the blood is small and can, for discussion purposes, beignored. Thus, for a patient waking up from a vapour anaesthetic, thegreater the rate of breathing, the more vapour is eliminated from thelungs. However, in anaesthetized patients breathing spontaneously,ventilation is often depressed as a result of combined effects ofresidual intravenously administered anaesthetic drugs, pain relievingdrugs (i.e. narcotics), the effects of surgery, as well as therespiratory depressant effect of the residual anaesthetic vapour itself.

[0018] Practically, there has been limited scope for intervention tohasten the process of eliminating vapour from the lung and thushastening the rate of emergence from the effects of vapour anaesthesia.

Proposals in Prior Art

[0019] 1. Artificial Ventilation

[0020] Manually or mechanically hyperventilating patients at the end ofsurgery is generally ineffective in shortening the time of recovery fromanaesthesia.

[0021] a) High ventilation using the circle anaesthetic circuit resultsin rebreathing of exhaled gases. These gases contain anaesthetic vapouras well as CO₂. The CO₂is eliminated by the CO₂ absorber in the circuit,but the exhaled anaesthetic vapour is returned to the patient.

[0022] b) The attempts at hyperventilation will result in a decrease inarterial PCO₂. The low arterial PCO₂ removes the stimulus to breathe,which in turn delays elimination of vapour (and may also preventadequate oxygenation of the blood). This is seldom practiced.

[0023] 2. Flushing the Circuit

[0024] High fresh gas flows in the circuit are inefficient in washingout the vapour from the circuits. The circle anaesthetic circuits havevolumes of approximately 8 L (not counting the patient's lung volume ofapproximately 2.5 L). At the maximum fresh gas flows on the oxygen flowmeter of 10 L/min, it would take about 4 minutes to wash out theanaesthetic vapour from the circuit alone!

[0025] 3. Stimulate Breathing

[0026] In the past, some anaesthetists tried to stimulate the patient'sbreathing by adding CO₂ to the breathing circuit. The rationale was toincrease the CO₂ concentration in the circuit, stimulate the patient tobreath harder until he managed to ventilate off the CO₂ and some of thevapour as well. This has largely been abandoned and has been labeled awasteful and dangerous practice.

[0027] a) It is wasteful for the reasons enumerated in 1 a and 1 b (videsupra). As well, the practice is wasteful in that extra CO₂ absorbingcrystals are consumed.

[0028] b) The technique may put a patient at risk if the patient cannotrespond to the extra CO₂ by increasing their ventilation. They willabsorb it and develop a high blood CO₂ concentration which can bedetrimental. The high CO₂ in the patient also causes them a good deal ofdistress on waking up as it makes them feel like they are not gettingenough air to breathe.

[0029] 4. Increase Ventilation, Keeping PCO₂ Constant

[0030] To increase ventilation without lowering PCO₂ requires adding CO₂to the circuit. This can be supplied from an external source or from thesubject's exhaled gas. All the presently described systems depend on aservo-controlled system, or feedback loop to regulate the amount of CO₂supplied to the patient. These devices are complex, cumbersome andexpensive. No such device has been reported used for hastening theelimination of anaesthetic vapour during recovery from anaesthesia.

[0031] With respect to 4 above, there are considerable limitations ofservo-controlled methods, both manual or automatic. These may bediscussed as follows:

[0032] 1. Input Signal

[0033] Whereas the parameter that we want to keep constant is thearterial PCO₂, feedback systems use the CO₂ concentration in the expiredgas, the so called end tidal PCO₂ (PETCO₂) as the input signal andendpoint. The PETCO₂ can be very different from the arterial PCO₂ inmany circumstances. Furthermore, changes in PETCO₂ may not correlatewith those in arterial PCO₂. This will result in PETCO₂ being aninappropriate input for the control of arterial PCO₂. For example, asmaller than usual breath decreases PETCO₂ (tending to increase arterialPCO₂), causing a servo-controller to respond with an inappropriateincrease in inspired CO₂.

[0034] 2. Gain

[0035] If, in an attempt to obtain fine control, the gain in aservo-control system is set too high, the response becomes unstable andmay result in oscillation of the control variable. Conversely, if thegain is set too low, compensation lags. Over-damping of the signalresults in a the response never reaching the target. To address theseproblems, servo-controllers require complex algorithms and expensiveequipment.

[0036] 3. Inherent Limitation

[0037] Servo-control systems work on the principle of detecting, andsubsequently attempting to correct for, changes in PETCO₂. Even underideal conditions, no such system can predict the size of an impending VTin a spontaneously breathing subject and thus deliver the appropriateCO₂ load.

[0038] As is apparent, people have tried to hasten the recovery ofpatients who have been anaesthetized and have made substantial effortsin this regard. However, they have been, for the most part, as seenabove, unsuccessful. The reason for the attempts is that the benefits offaster return to consciousness, the less the need for recovery care andthe less risk of nausea and post-operative respiratory complications.Thus the health care system will save substantial dollars. In thisregard, the cost to the health care system of operating room andrecovery area time is approximately $5.00 (Canadian Dollars) and $2.00(Canadian Dollars) per minute respectively. The total number ofanaesthetics given in North America is approximately 35,000,000/year(3.5 million and about 30 million in the United States), a conservativeestimate with as high as about 50,000,000/year. The North Americanestimate does not include Mexico or countries in Central America. Amodest average decrease in recovery time in the operating time and inthe recovery room of 5 minutes each can potentially result in billionsof dollars savings per year worldwide. In North America alone, theexpectation of saving 5 minutes in each of the operating room andrecovery area can amount to $1,000,000,000 in savings.

[0039] It is therefore an object of this invention to provide animproved breathing circuit or circuit components that can be added to astandard circle anaesthetic circuit to be used to hasten recovery ofpatients who have been administered vapour anaesthetics.

[0040] It is a further object of the invention to provide methods oftreatment using the said circuit and the use of the said circuit duringthe administration of vapour anaesthetics to hasten recovery of the saidpatients.

[0041] Further and other objects of the invention will be realized bythose skilled in the art reading the following summary of the inventionand detailed description of the embodiment thereof.

SUMMARY OF THE INVENTION

[0042] According to one aspect of the invention, there is provided a newbreathing circuit and components thereof that can, for example, be addedto a standard circle anaesthetic circuit for hastening the recovery ofpatients administered vapour anaesthetics.

[0043] In accordance with the invention, the said circuit and componentsthereof, when combined with the general anaesthetic circle circuit causethe administration of carbon dioxide gas to the patient to maintain thesame PCO₂ in the patient independent of the rate of ventilation (so longas the said rate of ventilation is greater than a control rate ofventilation) but permit the rate of anaesthetic vapour elimination fromthe lungs of the patient to vary directly as the total ventilation bythe patient, whether the patient is breathing normally or ishyperventilating. Thus, the vapour anaesthetic is eliminated from thelungs. However, the carbon dioxide is not eliminated from the lungs at arate greater than the resting rate of the patient or a predeterminedcontrol rate. (The predetermined rate of elimination of CO₂ may be setbased on the rate of administration of the fresh gas into the circuit asdiscussed below.)

[0044] Thus, according to another aspect of the invention, the simplebreathing circuit comprises components which together form the simplecircuit and comprise (a) an exit port from which the gases exit from thecircuit to the patient, (b) a non-breathing valve which constitutes aone-way valve permitting gases to be delivered to the exit port to bedelivered to the patient but which non-breathing valve when the patientbreathes into the exit port does not permit the gases to pass thenon-rebreathing valve into the portion of the circuit from which thegases are delivered but passes them to ambient or elsewhere, (c) asource of gas (which may be oxygen or air or other gases but does notcontain CO₂) (air contains physiologically insignificant amounts of CO₂)in communication with the non-breathing valve to be delivered throughthe valve to the patient, (d) a fresh gas reservoir in communicationwith the source of fresh gas flow for receiving excess gas not breathedby the patient from the source of gas and for storing same and when thepatient breathes and withdraws amounts of gas from the source of gasflow also enables the patient to receive gas from the fresh gasreservoir in which the gases have been stored, (e) a reserve gas supplycontaining CO₂ and other gases (usually oxygen) wherein the partialpressure of the CO₂ is approximately equal to the partial pressure ofthe CO₂ in the patient's mixed venous blood, for being delivered to thenon-rebreathing valve as required by the patient to make up that amountof gas required by the patient when breathing that is not fulfilled fromthe gases delivered from the source of gas flow and fresh gas reservoir,the said source of gas and fresh gas reservoir and reserve gas supplybeing disposed on the side of the valve remote from the exit port.

[0045] Preferably a pressure relief valve is in communication with thefresh gas reservoir, in the event that the fresh gas reservoir overfillswith gas so that the fresh gas reservoir does not break, rupture orbecome damaged in any way.

[0046] The reserve gas supply preferably includes a demand valveregulator so that where the additional gas is required, the demand valveregulator opens the communication of the reserve gas supply to thenon-rebreathing valve for delivery of the gas to the non-rebreathingvalve and where not required the demand valve regulator is closed andonly fresh gas flows from the source of fresh gas and from the fresh gasreservoir to the non-rebreathing valve. The source of fresh gas is setto supply fresh gas (non-CO₂-containing gas) at a rate equal to thedesired alveolar ventilation for the elimination of CO₂.

[0047] The basic concept underlying my approach is that when breathingincreases, the rate of flow of fresh gas (inspired PCO₂=0) from thefresh gas flow contributing to elimination of CO₂ is kept constant. Theremainder of the gas inhaled by the subject (from the reserve gassupply) has a PCO₂ equal to that of mixed venous blood, does notcontribute to a CO₂ concentration gradient between mixed venous bloodand alveolar gas, and thus does not contribute to elimination of CO₂. Ifthere is access to mixed venous blood (such as if a catheter is presentin the pulmonary artery, the mixed venous PCO₂ can be measured directly.If there is no possibility of measuring, then an estimation can be madefrom PETCO₂. PETCO₂ is determined by measuring the PCO₂ of expired usinga capnograph usually present or easily available in an operatingfacility by persons skilled in the art.

[0048] In effect, the device passively, precisely and continuouslymatches the amount of CO₂ breathed in by the patient to the amount oftotal breathing, thereby preventing any perturbation of the arterialPCO₂. This is opposed to servo-controllers which are always attemptingto compensate for changes. Persons skilled in the art, however, mayautomate the circuit by using a servo-controller or computer to monitorand deliver the amounts from the reserve gas supply.

[0049] According to another aspect of the invention, the new simplebreathing circuit is used to treat a patient to enable the patient torecover more quickly from, and to hasten the recovery of the patientafter, vapour anaesthetic administration.

[0050] According to another aspect of the invention, the use of the saidcircuit is made in the manufacture of a device to hasten the recovery ofpatients from administration of vapour anaesthetics.

[0051] According to another aspect of the invention, the use of saidcircuit is made to hasten the recovery of patients from vapouranaesthetics administration.

[0052] According to another aspect of the invention, a method oftreatment of an animal (for example, a person) is provided (such as toenable such animal to recover from vapour anaesthetics administration),the method comprising delivering to a patient gases which do not containCO₂ at a specified rate, and gases containing CO₂ to maintain the samePCO₂ in the animal independent of the rate of ventilation, at a rate ofventilation of the animal which exceeds the rate of administration ofthe gases which do not contain CO₂.

[0053] Therefore, when the rate of ventilation of the animal exceeds therate of delivery to the animal of non-CO₂-containing gases inhaled bythe animal, the CO₂-containing gases inhaled by the animal maintain thePCO₂ in the animal constant.

[0054] Thus, with respect to the use of the invention to eliminateanaesthetic vapour from the lungs, the total ventilation of combinedgases which includes the CO₂-containing, and non-CO₂ containing, gasesact to eliminate vapour from the lungs.

[0055] This circuit and methods of treatment can also be used for anycircumstance where one wants to dissociate the minute ventilation fromelimination of carbon dioxide such as respiratory muscle training,investigation of the role of pulmonary stretch receptors,tracheobronchial tone, expand the lung to prevent atelectasis, andcontrol of respiration and other uses as would be understood by thoseskilled in the art.

[0056] The circuit and methods of treatment may also be used by deep seadivers and astronauts to eliminate nitrogen from the body. It can alsobe used to treat carbon monoxide poisoning under normal baric or hyperbaric conditions. The fresh gas will contain 100% oxygen, and thereserve gas will contain approximately 6% CO₂ and approximately 94%oxygen. Neither the fresh gas nor the reserve gas supply will, in thiscase, contain nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0057]FIG. 1 illustrates schematically the nature of the simplebreathing circuit and components which enable the patient to recovermore quickly from vapour anaesthetics administration. The said deviceshown enables the PCO₂ to remain constant despite increase in minuteventilation which thereby permits faster elimination of the vapouranaesthetics.

[0058]FIG. 2 illustrates schematically portions of a standard circleanaesthetic circuit generally known to persons skilled in the art.

[0059]FIG. 3 illustrates schematically the simple breathing circuit inone embodiment added to the portions of the circle anaesthetic circuitshown schematically in FIG. 2, illustrating modifications of the circuitshown schematically in FIG. 1 for use with the generally known circuitshown in FIG. 2. (It would be clear to persons skilled in the art thatdepending upon the circuit used as the circle anaesthetic circuit,different modifications on the basic circuit shown in FIG. 1 will bemade).

[0060]FIG. 4A illustrates the structure shown in FIG. 3, now combinedwith the general structure shown in FIG. 2. (FIG. 3 shows themodifications made specifically to the structure of FIG. 1 to combine itwith the structure in FIG. 2 which is now shown in FIG. 4A.)

[0061]FIGS. 4B and 4C illustrate schematically close up portions of oneportion of the structure shown in FIG. 4A in different positions.

[0062]FIG. 5 graphs the VT (Tidal Volume) and PETCO₂.

[0063]FIG. 6 graphs traces of airway PCO₂ and VT.

[0064]FIGS. 7A, 7B and 8A and 8B graphically depict changes in PaCO₂ andPETCO₂.

DETAILED DESCRIPTION OF EMBODIMENTS

[0065] The circuit (FIG. 1) consists of a non-rebreathing valve (A)connected distally to two ports (C and D). The first port is connectedin parallel to a source of fresh gas (E) (which does not contain CO₂)and a fresh gas reservoir (F). A one-way pressure relief valve (G)prevents overfilling of the reservoir (F) by venting excess fresh gas.The second port (D) is connected via a one-way valve (H), to a source ofgas (containing CO₂) whose PCO₂ is equal approximately to that of themixed venous PCO₂. We call this the “reserve gas” (I). Non-rebreathingvalve A is connected to exit port J (from which the patient breathes).

[0066] Functional Analysis of Circuit Maintaining Constant PCO₂ withHyperventilation

[0067] When the minute ventilation “V” is less than or equal to thefresh gas flow “FGF” from (E), the subject inhales only fresh gas(non-CO₂-containing gas). When V exceeds FGF, the reservoir (F)containing fresh non-CO₂-containing gas empties first and the balance ofinhaled gas is drawn from the reserve gas (I) which contains CO₂. Thereserve gas is considered not to participate in CO₂ exchange ensuringthat the actual ventilation provided is limited by FGF. If the rate ofFGF is 5 L/minute and the patient breathes at 5 L/minute or less, thenthe patient will inhale only non-CO₂-containing gas that comes fromfresh gas flow sources (E and F). If minute ventilation exceed FGF, thedifference between minute ventilation and fresh gas flow is made up fromgas from reserve gas (I) which contains CO₂ at a concentration that doesnot provide a gradient for elimination of CO₂ in the patient.

[0068] Application of Circuit to Anaesthesia Circle Circuit

[0069] The Schematic of the Standard Anaesthetic Circle Circuit,Spontaneous Ventilation (FIG. 2)

[0070] When the patient exhales, the inspiratory valve (1) closes, theexpiratory valve (2) opens and gas flows through the corrugated tubingmaking up the expiratory limb of the circuit (3) into the rebreathingbag (4). When the rebreathing bag is full, the airway pressure-limiting(APL)valve (5) opens and the balance of expired gas exits through theAPL valve into a gas scavenger (not shown). When the patient inhales,the negative pressure in the circuit closes the expiratory valve (2),opens the inspiratory valve (1), and directs gas to flow through thecorrugated tube making up the inspiratory limb of the circuit (6).Inspiration draws all of the gas from the fresh gas hose (7) and makesup the balance of the volume of the breath by drawing gas from therebreathing bag (4). The gas from the rebreathing bag contains expiredgas with CO₂ in it. This CO₂ is extracted as the gas passes through theCO₂ absorber (8) and thus is delivered to the patient (P) without CO₂,(but still containing exhaled anaesthetic vapour, if any).

[0071] Modification of the Circuit (FIG. 3) to Allow Hyperventilation ofPatients Under Anaesthesia

[0072] The modified circuit consists of

[0073] 1. a circuit which acts functionally like a standard selfinflating bag (such as made by Laerdal) consisting of

[0074] a) a non rebreathing valve, such as valve #560200 made byLaerdal, that functions during spontaneous breathing as well as manuallyassisted breathing (9);

[0075] b) an expired gas manifold, such as the Expiratory Deviator#850500, to collect expired gas (10) and direct it to a gas scavengersystem (not shown) or to the expiratory limb of the anaesthetic circuit(FIG. 4);

[0076] c) a self inflating bag (11) whose entrance is guarded by a oneway valve directing gas into the self inflating bag (12).

[0077] 2. a source of fresh gas, (i.e. not containing vapor) e.g. oxygenor oxygen plus nitrous oxide (13) with a flow meter (22).

[0078] 3. a manifold (14) with 4 ports:

[0079] a) a port (15) for input of fresh gas (13);

[0080] b) a port (16) for a fresh gas reservoir bag (17);

[0081] c) a port to which is attached a one way inflow valve that openswhen the pressure inside the manifold is 5 cm H₂O less than atmosphericpressure, such as Livingston Health Care Services catalog part #9005,(18) (assuring that all of the fresh gas is utilized before opening);

[0082] d) a bag of gas (19) whose PCO₂ is equal approximately to that ofthe mixed venous PCO₂ connected to inflow valve (18) (Alternatively, thevalve and gas reservoir bag can be replaced by a demand regulator, suchas Lifetronix MX91120012, similar to that used in SCUBA diving, and acylinder of compressed gas);

[0083] e) a port to which is attached a one way outflow valve (20), suchas Livingston Health Care Services catalog part #9005, that allowsrelease of gas from the manifold to atmosphere when the pressure in themanifold is greater than 5 cm H₂O.

[0084] Method of Operation in an Anaesthetic Circuit (FIG. 4A)

[0085] The distal end of the nonrebreathing valve (Laerdal type) (9), isattached to the patient.

[0086] The proximal port of the nonrebreathing valve is attached to a 3way respiratory valve (21) which can direct inspiratory gas either fromthe circle anaesthetic circuit (FIG. 4B)or from the new circuit (FIG.4C). The expiratory manifold (10) of the self inflating bag's nonrebreathing valve is attached to the expiratory limb of the anaestheticcircuit (3). Regardless of the source of inspired gas, exhalation isdirected into the expiratory limb of the anaesthetic circuit.

[0087] To maximize the elimination of anaesthetic vapour from thepatient's lungs, the 3-way respiratory stopcock is turned such thatpatient inspiration is from the new circuit (FIG. 4C). Thus inspired gasfrom the very first breath after turning the 3-way valve onward containsno vapour, providing the maximum gradient for anaesthetic vapourelimination.

[0088] An increased breathing rate will further enhance the eliminationof vapour from the lung. If breathing spontaneously, the patient can bestimulated to increase his minute ventilation by lowering the FGF (22)thereby allowing the PCO₂ to rise. Using this approach the PCO₂ willrise and plateau independent of the rate of breathing, resulting in aconstant breathing stimulus. All of the ventilation is effective ineliminating vapour.

[0089] If the patient is undergoing controlled ventilation, he can alsobe hyperventilated with the self inflating bag (11). In either case, thepatient's PCO₂ will be determined by the FGF (22). As long as the FGFremains constant the PCO₂ will remain constant independent of the minuteventilation.

[0090] To illustrate the effectiveness of the circuit we performed anumber of tests with respect to humans and dogs. The humans werebreathing spontaneously and the dogs were mechanically ventilated.

[0091] Human Subjects

[0092] After obtaining institutional ethics board approval and informedconsent, four healthy subjects aged 19-25 y breathed through the circuitby means of a mouth piece while wearing nose clips. During normalbreathing, the FGF was set equal to V by adjusting the FGF such that thebag containing fresh gas just emptied at the end of each inhalation.Subjects were then instructed to breathe maximally (“breathe as hard asyou can”) for 3 min. Flows were recorded by means of a Pitot tube(Voltek Enterprises, Willowdale Canada) and the signal integrated toobtain volume. CO₂ was sampled continuously at the mouthpiece (MedicalGas Analyzer LB-2, Sensormedics Corp., Anaheim, Calif.). Analog signalswere digitized at 60 samples·S⁻¹ and recorded using data acquisitionsoftware (WINDAQ/200, DATAQ instruments, Inc. Akron Ohio).

[0093] Studies in Dogs

[0094] Following institutional ethics board approval, 6 mongrel dogs ofeither sex weighing 20-25 kg were anaesthetized with methohexital (5-7mg·kg⁻¹ for induction followed by 150-300 mg·kg⁻¹·min⁻¹) and intubated.Adequacy of anaesthetic depth was deduced from the eye lash reflex, lackof spontaneous movements, and stable heart rate and blood pressure. Acatheter was placed in the femoral artery for monitoring blood pressureand periodic sampling of blood for gas analysis. The dogs wereventilated with a conventional mechanical piston ventilator (HarvardApparatus model 618, South Natick, Mass.). For each dog, an inflationvolume (VT) of 400 ml and a frequency (f) of 10 min⁻¹ (duty cycle, 0.5)were used. All dogs were ventilated to just below their apneicthresholds (by increasing VT about 50 mL) so that they made norespiratory efforts. Tidal CO₂ was sampled continuously (Ametek, ThermoxInstruments Division, Pittsburgh, Pa.) at the proximal end of theendotracheal tube. Flow was measured with a pneumotachograph (Vertekseries 47303A, Hewlett-Packard) and the signal integrated to obtainvolume. Analog signals were digitized at 17 samples·S⁻¹ and recordedusing the same data acquisition software as that used in studies onhuman subjects.

[0095] Because of differences in initial PaCO₂s among dogs (reflectingindividual sensitivities to CO₂, differences in anaesthetic levels, ordifferences in VT/body weight ratio), the CO₂ concentration in thereserve gas was arbitrarily adjusted for each dog to 1.5 ±0.5% above itsFetCo₂ to approximate the mixed venous PCO₂ (PvCO₂) (see Table II). Toallow greater flexibility in setting the concentration of CO₂ in thereserve gas, the circuit was modified by replacing the demand valve witha one-way PEEP (positive end expiratory pressure) valve and the cylinderwith a bag containing premixed gas. This circuit is functionallyidentical to that used in studies on humans. The circuit was connectedto the intake port of the ventilator. Under control conditions, FGF wasadjusted so that the fresh gas reservoir just emptied during eachventilator cycle; this end point was confirmed by a slight rise in FICO₂above zero. After a steady-state had been reached (difference <1.5 mm Hgin two successive PaCO₂ ′s taken 5 minutes apart), VT was increased at 5minute intervals from 400 to 600 to 900 to 1200 mL. In a second trial ata fixed VT (approximately 400 mL) and fixed FGF, f was increased at 5minute intervals from 10 to 14 to 18 to 22 min⁻¹. A blood sample for thedetermination of blood gases was drawn from the femoral artery at thebeginning and end of each 5 min interval.

[0096] All data are expressed as means ± standard deviation. We testedfor significant differences using one- or two-way ANOVA with post hocanalysis where appropriate. A p value less than 0.05 was consideredsignificant.

[0097] Results

[0098] Human Subjects

[0099]FIG. 5 presents the VT and PETCO₂ of subject 1 during 3 min ofmaximal ventilatory effort. Results for all subjects are summarized inTable III; data represent average values for 10 breaths at 0 (the onsetof hyperventilation), 1.5 and 3 min. PETCO₂ did not change significantlyfrom control values throughout the course of hyperventilation (p=0.08,ANOVA). There was considerable variability in V and breathing patternsbetween subjects but individual subjects tended to sustain a particularbreathing pattern throughout the run.

[0100] Dogs

[0101]FIG. 6 presents traces of airway PCO₂ and VT for dog #5 duringchanges in f or VT. FIGS. 7 and 8 show the changes in PaCO₂ and thePETCO₂ in all dogs during changes in for VT. Increases in f did notsignificantly affect mean PaCO₂ or PETCO₂ (p=0.28 and p=0.11,respectively; ANOVA). Increases in VT decreased mean PaCO₂ from controlonly at VT Of 1200 mL (p=0.01); in contrast, changes in VT did notaffect mean PETCO₂ (p=0.25). The mean absolute change in PaCO₂ betweencontrol and the highest ventilation was 2.2±1.8 mmHG (range 0.4 to 4.8)for f and 3.4±2.3 mmHG (range 0.4 to 4.8) for f and 3.4±2.3 mmHG (range0.4 to 5.6) for VT.

[0102] COMMENTARY

[0103] The system minimized decreases in PETCO₂ over a wide range ofventilation (56 to 131 L min⁻¹)and breathing patterns, inhyperventilating human subjects and in mechanically hyperventilated dogs(4 to 12 L min⁻¹). The variability in PaCO₂ in the hyperventilated dogs,although small, may have been due to a) imprecise matching of reservegas PCO₂ to the dog's PvCO₂s;b) prolonged duration of the maneuver indogs (>15 min versus 3 min for human subjects) and c) the extent ofhyperventilation (see below). In addition, the different levels ofventilation may have induced changes in systemic and pulmonary bloodflow (ventilation-perfusion matching, physiological and anatomical deadspace), thereby affecting PaCO₂ and PvCO₂. Despite these sources ofvariability, the range over which PaCO₂ varied in my studies in dogs wassimilar to those reported in studies utilizing more complex equipment(see Table 1).

[0104] Conventional servo-controlled techniques designed to preventchanges in PCO₂ with hyperpnea are less affected by changes in CO₂production than the circuit; however, they have other limitations. Theassumption that detected changes in PETCO₂ are due to a change in PaCO₂is not always warranted (14). Small changes in ventilatory pattern can‘uncouple’ PETCO₂ from PaCO₂, resulting in PETCO₂ being an inappropriateinput for the control of PaCO₂. For example, a smaller VT decreases VA(which tends to increase PaCO₂) but will also decrease PETCO₂, causing aservo-controller to respond with an inappropriate increase in inspiredCO₂. Even under ideal conditions, a servo-controlled system attemptingto correct for changes in PETCO₂ cannot predict the size of an impedingVT in a spontaneously breathing subject and thus deliver the appropriateCO₂ load. If in an attempt to obtain fine control the gain in aservo-control system is set too high, the response becomes unstable andmay result in oscillation of the control variable (11). Conversely, ifthe gain is set too low, compensation lags (9). Over-damping of thesignal results in a response never reaching the target. To address theseproblems, servo-controllers require complex algorithms (16) andexpensive equipment.

[0105] When CO₂ production is constant, the circuit has the theoreticaladvantage over servo-controlled systems in that it provides passivecompensation for changes in V. This minimizes changes in VA, pre-emptingthe need for subsequent compensation. Maintenance of a nearly constantVA occurs even during irregular breathing, including brief periods whenV is less than the FGF. Under this circumstance, excess FGF is stored inthe fresh gas reservoir and subsequently contributes to VA whenventilation exceeds FGF.

[0106] When CO₂ production increases during hyperventilation, as wouldoccur with increased work of breathing or exercise, my method requiresmodification. To compensate, additional VA can be provided either byincreasing FGF or by lowering the PCO₂ of the reserve gas below thePvCO₂, as expressed in the following equation:

V A =FGF+(V−FGF)(PvCO₂−reserve gas PCO₂)

[0107] Because spontaneously breathing subjects had such variable Vduring hyperventilation, compensating for the CO₂ production bymodifying FGF would have required constant adjustment. We thereforechose to decrease the PCO₂ of the reserve as to establish aconcentration gradient between the PCO₂ of the reserve gas and thePvCO₂; when this is constant, VA is a function of V. We found that, overthe wide range of V exhibited by the subjects, a concentration of 5.5%CO₂ in the reserve gas (instead of 6.5% which corresponds to a PvCO₂ of46 mmHG) provided the optimal gradient to compensate for increases inCO₂ production resulting from increased work of breathing.

[0108] I therefore have described a simple circuit that disassociates VAfrom V. It passively minimizes increases in VA that would normallyaccompany hyperventilation when CO₂ production is constant. It can bemodified to compensate for increases in CO₂ production. The circuit mayform the basis for a simple and inexpensive alternative toservo-controlled systems for research and may have therapeuticapplications. TABLE II Weight Initial FETCO₂ Bag FCO₂ Dog # (kg) (%) (%)1 22 5.3 7.0 2 20 4.6 6.6 3 20 7.1 9.0 4 24 7.3 9.0 5 25 5.5 6.9 6 206.0 7.2

[0109] TABLE III Time Subject # Control 0 1.5 3 End Tidal PCO₂ (mmHg) 140.3 33.6 34.9 35.6 2 36.6 30.9 28.1 28.0 3 42.0 42.5 43.2 42.7 4 41.034.5 38.8 38.8 Frequency (min⁻¹) 1 57 50 47 2 89 87 88 3 31 30 30 4 149130 127 Tidal Volume 1 2.30 2.49 2.58 2 0.85 0.72 0.63 3 2.60 2.64 2.264 0.78 0.62 0.60 Minute Ventilation (L · min⁻¹) 1 131 124 118 2 75 63 563 80 78 68 4 117 80 76

[0110] While the foregoing provides a detailed description of apreferred embodiment of the invention, it is to be understood that thisdescription is illustrative only of the principles of the invention andnot limitative. Furthermore, as many changes can be made to theinvention without departing from the scope of the invention, it isintended that all material contained herein be interpreted asillustrative of the invention and not in a limiting sense.

What is claimed:
 1. A breathing circuit and components thereof forcausing the administration of carbon dioxide gas to the patient tomaintain the same PCO₂ in the patient independent of the rate ofventilation (so long as the said rate of ventilation is greater than acontrol rate of ventilation).
 2. The circuit of claim 1 wherein thecircuit permits the rate of anaesthetic vapour elimination from thelungs of the patient to vary directly as the total ventilation by thepatient, whether the patient is breathing normally or ishyperventilating.
 3. The circuit of claim 1 wherein the circuit permitsthe rate of nitrogen elimination from the lungs of the patient to varydirectly as the total ventilation by the patient, whether the patient isbreathing normally or is hyperventilating.
 4. A breathing circuitcomprises components which together form the simple circuit and comprise(a) an exit port from which the gases exit from the circuit to thepatient, (b) a non-rebreathing valve which constitutes a one-way valvepermitting gases to be delivered to the exit port to be delivered to thepatient but which non-breathing valve when the patient breathes into theexit port does not permit the gases to pass the non-rebreathing valveinto the portion of the circuit from which the gases are delivered butpasses them to ambient or elsewhere, (c) a source of gas (which may beoxygen or air or other gases but does not contain CO₂ in communicationwith the non-breathing valve to be delivered through the valve to thepatient, (d) a fresh gas reservoir in communication with the source offresh gas flow for receiving excess gas not breathed by the patient fromthe source of gas and for storing same and when the patient breathes andwithdraws amounts of gas from the source of gas flow also enables thepatient to receive gas from the fresh gas reservoir in which the gaseshave been stored, (e) a reserve gas supply containing CO₂ and othergases wherein the partial pressure of the CO₂ is approximately equal tothe partial pressure of the CO₂ in the patient's mixed venous blood, forbeing delivered to the non-rebreathing valve as required by the patientto make up that amount of gas required by the patient when breathingthat it not fulfilled from the gases delivered from the source of gasflow and fresh gas reservoir, the said source of gas and fresh gasreservoir and reserve gas supply being disposed on the side of the valveremote from the exit port.
 5. The circuit of claim 4 further comprisinga pressure relief valve is in communication with the fresh gasreservoir, in the event that the fresh gas reservoir overfills with gasto that the fresh gas reservoir does not break, rupture or becomedamaged in any way.
 6. The circuit of claim 4 or 5 wherein the reservegas supply includes a demand valve regulator so that where theadditional gas is required, the demand valve regulator opens thecommunication of the reserve gas supply to the non-rebreathing valve fordelivery of the gas to the non-breathing valve and where not requiredthe demand valve regulator is closed and only fresh gas flows from thesource of fresh gas and from the fresh gas reservoir to thenon-rebreathing valve.
 7. The use of the circuit of claim 1, 2, 3, 4, 5or 6 is made in the manufacture of a device to hasten the recovery ofpatients from administration of vapour anaesthetics.
 8. The use of thecircuit of claim 1, 2, 3, 4, 5 or 6 to hasten the recovery of patientsfrom vapour anaesthetics administration.
 9. A method of treatment of ananimal to recover from vapour anaesthetics administration, the methodcomprising delivering to a patient gases which do not contain CO₂ at aspecified rate, and gases containing CO₂ to maintain the same PCO₂ inthe animal independent of the rate of ventilation, at the rate ofventilation of the animal which exceeds the rate of administration ofthe gases which do not contain CO₂.
 10. The method of claim 9 whereinthe animal is a person.
 11. The method of claim 9 or 10 wherein theanimal is enabled to recover from vapour anaesthetics administration,the method comprising delivering to a patient gases which do not containCO₂ at a specified rate, and gases containing CO₂ to maintain the samePCO₂ in the animal independent of the rate of ventilation, at the rateof ventilation of the animal which exceeds the rate of administration ofthe gases which do not contain CO₂.